The invention relates to tunable optical devices.
Fiber optic communication systems utilizing wavelength division multiplexing (WDM) are well known. Conventional WDM fiber optic communication systems include several optical components, such as multiplexers, demultiplexers, star couplers, tunable optical filters, multiwavelength optical transmitters with tunable semiconductor lasers, wavelength routers, wavelength shifters, and optoelectronic regenerators or amplifiers.
Multiplexers are conventionally used to combine the output of several transmitters and launch that output into an optical fiber. Demultiplexers are conventionally used to split received multichannel signals into individual bands destined to different receivers. Star couplers mix the output of several transmitters and broadcast the mixed signal to multiple receivers. Tunable optical filters filter out one band at a specific wavelength to isolate that band from the multitude of other possible bands. The specific wavelength of the band can be changed by tuning a primary waveguide of the optical filter. Multiwavelength optical transmitters with tunable semiconductor lasers have a wavelength that can be tuned over a few nanometers. Wavelength routers can distribute a WDM optical signal to different ports. Wavelength shifters switch the band wavelength, and optoelectronic regenerators or amplifiers are used for boosting an optical signal.
In WDM fiber optic communication systems, broad optical signals made up of various bands are transmitted over long distances. The optical signals have a certain intensity which has to be maintained at predetermined intervals along the transmission path. This can be accomplished through the use of amplifiers set up along the transmission path. Further, each of the various bands has its own intensity, and it is often necessary to equalize, or flatten, the intensity of the various bands to prevent large differences in optical power accumulating after each of the amplifiers. One conventional method of providing amplifier gain equalization is based on controlling tunable optical filters.
Tunable optical filters have certain desired properties. One of the desired properties is a wide tuning range which maximizes the number of band selections possible. Other desirable properties include negligible crosstalk for avoiding interference from adjacent bands, fast tuning speed for minimizing access time, small insertion loss, polarization insensitivity, stability against environmental changes, and low cost. Known tunable optical filters include Fabry-Perot filters, Mach-Zehnder filters, grating-based Michelson filters, and acousto-optic filters.
Some optical filters, such as a Mach-Zehnder filter, take in a broad optical signal having a plurality of channels each with its own intensity. One portion of the optical signal is segregated off from the other and sent down a bypass path, while the other portion of the optical signal is transmitted through a primary path. The optical signal portion transmitted through the bypass path is not altered, while the optical signal portion transmitted through the primary path is separated into individual bands via a demultiplexer. Each of the separated out bands is then phase-adjusted to alter its transmissivity through the entire device. Typically, the bands are tuned by way of thermal devices, such as heaters.
After tuning the various individual bands, the bands are then all run through a multiplexer, which recombines the bands which have had their phases adjusted. The recombined signal portion differs from the original optical signal portion which was transmitted to the demultiplexer in that the intensity of each of the individual bands has been adjusted. After recombination, the primary and bypass paths are coupled at a second coupler. The recombination of the optical signal portion from the bypass path and the now altered optical signal portion from the primary path may result in certain bands being in phase, certain bands being out of phase, and thereby filtered out, and certain bands being partially in phase, and thereby partially filtered out.
The recombined and altered signal exiting from the second coupler is then transmitted along a preselected signal path to its final destination or on to another optical filter. As the recombined and altered signal exits the second coupler it is transmitted to a sensor which taps off and analyzes a small portion, such as one percent, of the signal. If the sensor detects that the recombined and altered signal is not within a predetermined range of intensity, the sensor sends a signal to a controlling element, such as a microprocessor controller, which in turn sends signals to the various heaters to adjust one or more of the heaters so as to further tune one or more bands.
A disadvantage of these tunable optical filters is that they have a fixed dynamic range and a fixed minimum insertion loss. Dynamic range is the degree to which the intensity of individual bands in an optical signal and/or the intensity of the optical signal as a whole can be adjusted. Insertion loss is the loss of power from inserting a component into an optical path. Over time, as a fiber optic communication system performance changes, the tunable optical filter sometimes cannot be adjusted enough to compensate for the performance changes due to the fixed dynamic range, and it must be exchanged for a new filter that has an appropriate dynamic range for the changed performance.
The invention provides a thermo-optic device adapted to vary its dynamic range. The device includes a substrate, an input port for receiving an input signal, first and second planar waveguides situated within the substrate and adapted to receive a portion of an input signal, first and second coupling regions at which the waveguides are coupled, an output port coupled to the second coupling region, a first signal line for receiving a control signal, and a first heater strip positioned at the first coupling region. The first heater strip is responsive to a received first control signal on the signal line to alter the temperature characteristic of the first coupling region. In one aspect, a second heater strip at the second coupling region is responsive to a received second control signal on a second signal line to alter the temperature characteristic of the second coupling region. By controlling the first coupling region, and in the aforementioned one aspect, the first and second coupling regions, the dynamic range of the thermo-optic device can be altered as necessary.
The invention also provides a fiber optic communication system which includes one or more thermo-optic devices adapted to vary their dynamic range. The system includes an input signal generator for generating an input optical signal, an output receiver; and a thermo-optic device coupled between the input optical signal generator and the output receiver. The thermo-optic device includes a first heater strip positioned at a first coupling region. The first heater strip is responsive to a received first control signal on a first signal line to alter the temperature characteristic of the first coupling region and thus the dynamic range of the thermo-optic device.
The invention further provides a method for using a thermo-optic device. The method includes providing an optical signal having a plurality of multiplexed optical bands, splitting the optical signal into first and second portions which respectively travel through a first planar waveguide and a second planar waveguide, demultiplexing the first portion of the optical signal transmitted along the first planar waveguide into individualized bands, altering the phase of one or more of the individualized bands, multiplexing together the individualized bands, combining the portions of the input signal transmitted along the first and second planar waveguides to produce an output signal, and adjusting the coupling ratio of the first and second portions of the optical signal in response to the output signal.
These and other advantages and features of the invention will be more readily understood from the following detailed description of the invention which is provided in connection with the accompanying drawings.